1. Field of the Invention
This invention relates to a process for preparing polymer particles, and, more particularly, to a process for preparing polymer particles in which seed particles are used for preparing polymer particles by emulsion seed polymerization or soap-free seed polymerization.
2. Description of the Background Art
Synthesis of polymer emulsions in an aqueous system is conventionally performed by emulsion polymerization, soap-free polymerization, or suspension polymerization. In these polymerization methods polymer particles are added to the polymerization reaction system before polymerization is initiated. Such a method is called seed polymerization. Seed polymerization includes multiple stage polymerization in which the seed reaction is carried out successively in the same reaction vessel following the preparation of seed particles. Seed polymerization has an advantage in that the particle size of the seed polymers can be comparatively freely controlled, since the number of particles in the polymerization system can be controlled by the seed. For this reason, this method is widely used.
The amount of seed polymer particles used in seed polymerization is smaller than the amount of the produced polymer particles, and, in many cases, the seed polymer particles are included within the produced polymer particles. For these reasons, the seed polymer particles are not believed to greatly influence the characteristics of the produced polymer particles. At the present time, investigations into seed polymer particles are not sufficient.
Few technological reports have heretofore been published about the influence of seed polymer particles on the molecular weight or on the degree of crosslinking, which are the most important properties of the produced polymers.
Particles produced by emulsion polymerization or soap-free polymerization are commonly used as seeds in conventional seed polymerization. These seed particles usually have a weight average molecular weight in the range of one hundred thousand to several millions and a gel content of several tens of percent to zero percent.
U.S. Pat. No. 4,186,120, Japanese Patent Laid-open Nos. 97582/1979 and 189413/1988 disclose the seed polymerization processes in which low molecular weight seed particles are used. Both processes employ the one-time polymerization method. That is to say, in both processes low molecular weight seed particles are used and monomers are charged at one time. The monomers are then absorbed to seed particles and polymerized.
On the other hand, Japanese Patent Laid-open Nos. 225208/1986, 22509/1981, and 72714/1988 disclose seed polymerization processes in which high crosslinked seed polymer particles are used. Since the crosslinked seed particles used in these processes do not dissolve into monomers and thus do not melt, they are stable and produce less agglomerated substances during polymerization.
Japanese Patent Laid-open No. 107089/1974 and No. 106586 disclose processes for producing composite particles formed of different types of polymers, in which an urethane polymer or an epoxy oligomer is used as seed particles for polymerizing vinyl monomers which are quite different from the urethane polymer or epoxy oligomer. The processes had a drawback of poor polymerization stability due to the great difference between the seed particles and the polymerized polymers.
In view of this situation, the present inventors have undertaken studies in order to achieve a great improvement over conventional processes in the stability during polymerization and to expand the types of monomers, the monomer composition, and the additives which can be used in the polymerization.
There are generally undue restrictions as to the types of monomers, their compositions, the types of stabilizers, and their amounts used for the emulsion polymerization due to its polymerization mechanism. Specifically, polymerization of highly crosslinked monomers, polymerization of monomer compositions having a higher water-solubility, polymerization using a smaller amount of stabilizers, polymerization using large size particles, and polymerization under a high solid component concentration were substantially unfeasible because of poor polymerization stability. Development of a polymerization process free from this drawback and exhibiting high polymerization stability has therefore been desired.